1. Field of the Invention
This invention relates to short range communications using surface acoustic wave (SAW) expanders and compressors.
2. Description of the Prior Art
SAW technology is well known for its excellent radio frequency (RF) performance, low cost and small size. SAW is a passive thin film technology that does not require any bias current in order to function. SAW expanders and compressors have been used in RADAR applications for many years.
The basic “building block” of SAW expanders and compressors is the interdigital transducer (IDT) such as shown in FIG. 1. An IDT 10 is a series of thin metal strips or “fingers” 12 fabricated on a suitable piezoelectric substrate 14. One set of fingers is connected to an input/output terminal 16, while the opposite set of fingers is connected to another terminal 18. In single-ended IDTs, terminal 18 is grounded. For differential input signals however, terminal 18 is a pulse input/output terminal. Spacing “W” between IDT segments is adjusted to conform to the desired chip period of the coded sequence. When excited by a narrow pulse at terminal 16, the IDT generates a coded output SAW which propagates in both directions perpendicular to the fingers 12. If a similarly coded SAW impinges on the fingers 12, then an autocorrelation function is performed and a peak, with associated side lobes, is generated at terminal 16. These abilities of SAW expanders and compressors are well known in the prior art, having been demonstrated for example in Edmonson, Campbell and Yuen, “Study of SAW Pulse Compression using 5×5 Barker. Codes with Quadraphase IDT Geometries”, 1988 Ultrasonics Symposium Proceedings, Vol. 1, 2-5 Oct. 1988, pp. 219-222.
Thus the structure shown in FIG. 1 can operate as both a SAW expander, generating a SAW output from a single pulse input, and a SAW compressor, generating a single pulse or peak output from a SAW input. Terminal 16, as well as terminal 18 in differential IDTs, is both a pulse input terminal and a pulse output terminal. Conversion of an output SAW into an electrical signal for further processing in conventional communications circuits and subsequent transmission through an antenna is accomplished by adding a transmit IDT 24, aligned with the IDT 22, as shown in FIG. 2. Both IDTs can be fabricated on the same substrate 14. A SAW output from IDT 22 is converted into an electrical signal by TX IDT 24. A SAW receiver would have the same structure as in FIG. 2. A signal input to a receive IDT from receiver processing circuitry would be converted to a SAW which is input to IDT 22. Like the IDT 22, the TX IDT 24 may be a differential IDT, wherein the grounded lower terminal would be a pulse output terminal.
The geometry of adjacent IDT fingers 12 is shown in FIG. 3, where Tf is the width of a metallized finger 12 and Ts is the width of the space between the fingers 12. In typical designs both Tf and Ts are equal to a quarter of a wavelength, λ/4. Since wavelength is inversely proportional to frequency of operation, higher frequency IDTs require thinner fingers 12 located in close proximity to each other, which complicates fabrication and reduces fabrication yields. For example, for a typical SAW system operating in the Industrial, Scientific and Medical (ISM) band at 2.4 GHz the λ/4 dimension could be in the order of 0.425 microns, depending upon the substrate chosen.
Previous communications system designs sought to overcome these manufacturing difficulties by using lower frequency SAW expanders and compressors having larger and further spaced fingers in conjunction with mixers and local oscillators, as shown in FIG. 4. In the typical prior art communication system 30, the lower frequency 266 MHz signal generated by transmit IDT 20 is up-converted in mixer 34, which receives a 734 MHz signal from local oscillator 36. The output from mixer 34 is filtered in high pass filter 38 to produce a 1 GHz signal which is transmitted through antenna 40. On the receive side, the process is reversed in antenna 42, mixer 44, low pass filter 46 and receive compressor IDT 20′. As discussed above, transmit IDT 20 and receive IDT 20′ have similar structure. Undesirably, the mixers 34 and 44, oscillator 36 and filters 38 and 46 from the communications system 30, result in additional cost, power consumption, occupation in space and a much complex system than is desired for low-cost, low power, short range communication systems. Therefore, there remains a need in the art to reduce the number of components in such a communication system.
High-frequency communication techniques involving more conventional non-SAW based circuits and systems also exist. Bluetooth™ wireless technology is one such prior art example. Bluetooth is a de facto standard, as well as a specification for small-form factor, low-cost, short range radio links between mobile PCs, mobile phones and other portable wireless devices. The current Bluetooth short range communications specification operates in the 2.4 GHz (ISM) band; however, in reality the technology for mobile communication devices involves undesirable high cost, substantial power consumption and relatively complex hardware.